1. Field of the Invention
The present invention relates to a method of fabricating a Bi-Sr-Ca-Cu-O superconductive film and, in particular, to a superconductive film of an a-axis or b-axis preferential orientation with respect to a substrate surface, which is very effective for Josephson devices, three-terminal devices or the like.
2. Description of the Related Art
Because of the high superconductivity critical temperature (Tc), oxide-type superconductive films are expected to be applied to electronic devices, such as transistors and Josephson junction devices. However, the superconductivity of an oxide-type superconductive film exhibits a strong anisotropy, so that it requires a film orientation control. In the case of device application, in particular, a device structure whose ab crystal surface with a large coherence length (.zeta.) allows a flow of electric current is advantageous from the viewpoint of the properties of the device to be obtained. For this purpose, it is important that the film should be of a-axis or b-axis orientation with respect to the substrate surface.
Conventionally known methods of fabricating oxide-type superconductive films include the PVD (physical vapor deposition) method and the CVD (chemical vapor deposition) method. The lower limit of the film formation speed in the PVD method is relatively high as compared to that in the CVD method, and a reduction in speed in the PVD method will result in the irregular film formation speed and the film composition, thereby making it difficult to obtain an ultra-thin film which is considered producible at low film formation speed (Applied Physics Letters. Vol., 53, No. 7, 1988, pp. 624).
Generally speaking, it is possible with the CVD method to maintain the saturated vapor of the material compound stable within a fixed temperature range. Further, before they are caused to react so as to effect film formation on a substrate, the vapor and the oxidizing gas are separated from each other, so that it is possible to cause the requisite reaction to take place exclusively on the substrate (Journal of Applied Physics, Vol. 67, No. 3, 1990, pp. 1562). In this respect, the method is superior to the PVD method, in which it is difficult to control the reaction between the material gas and the oxidizing gas and the material vapor concentration in the system is unstable.
Thus, it is more advantageous from the industrial point of view, to fabricate oxide-type superconductive films by the CVD method.
As stated above, when using oxide-type superconductive films for various types of devices, an a-axis or b-axis film orientation is desirable. However, because of their strong anisotropy in crystal structure, oxide-type superconductive films have been inclined to c-axis orientation, which is particularly true for the Bi-Sr-Ca-Cu-O-type superconductive films.
There have conventionally been reported examples of a Bi-Sr-Ca-Cu-O-type superconductive film in which an a-axis or b-axis preferential growth is effected by sputtering, as well as examples of a Y-Ba-Cu-O-type superconductive film in which an a-axis or b-axis preferential growth is effected by the CVD method. However, there has been provided as yet no Bi-Sr-Ca-Cu-O-type superconductive film of a-axis or b-axis orientation prepared by the CvD method.
Thus, there has been a request for a formation of a Bi-Sr-Ca-Cu-O-type superconductive film of a-axis or b-axis orientation with respect to the substrate surface by the CVD method.